Field of the Invention
The present invention relates to a technology for wirelessly transmitting data in a multicast manner.
Description of the Related Art
Conventionally, when a video receiving terminal selects and displays videos transmitted from a plurality of video transmission terminals, a video signal switching terminal (a switcher) is arranged between the video receiving terminal and the video transmission terminals, the video receiving terminal and the video signal switching terminal are connected to each other using a cable, the video signal switching terminal and the video transmission terminals are connected to each other using cables, and the video signal switching terminal is controlled, so that the selection of videos is performed. When the number of video receiving terminals and video transmission terminals used increases, there are problems that the wiring of cables and the configuration of the video signal switching terminal are complicated and it is not possible to quickly cope with a layout change in the video receiving terminals and the video transmission terminals.
In recent years, with the improvement of a wireless transmission technology, it is possible to wirelessly transmit videos from a video transmission terminal to a video receiving terminal. In this way, it is possible to cope with the aforementioned problems and realtime transmission from one video transmission terminal to a plurality of video receiving terminals has become possible.
As a scheme of efficiently transmitting data to a plurality of video receiving terminals, a multicast transmission scheme has been known. Since the conventional multicast transmission scheme is a scheme of one-sidedly transmitting data from a video transmission terminal to video receiving terminals without performing reception confirmation, it is a transmission scheme with low reliability and is largely affected by a communication error.
On the other hand, a transmission scheme in which reliability is improved by performing reception confirmation also in the multicast transmission has been proposed. According to this scheme, when no reception is confirmed through the reception confirmation, video data is retransmitted, so that it is possible to improve the reliability of communication.
FIG. 26 illustrates an aspect in which video data is transmitted from a video transmission terminal to n (n>1) video receiving terminals (illustrated as video receiving terminals 1 to n in FIG. 26). The right direction of FIG. 26 is a direction in which time passes. The video data transmitted from the video transmission terminal is divided into a plurality of pieces of video data, and each divided piece of video data is wirelessly transmitted within the time of one frame in a time division scheme and in a multicast manner. Video receiving terminals having received the video data transmit reception responses (ACK) to the video transmission terminal in response transmission periods (S1, S2, . . . , and Sn), which have been assigned to the respective video receiving terminals, in a reception response period. The video transmission terminal receives the reception responses from the video receiving terminals and performs reception confirmation.
In a multicast transmission scheme with reception confirmation, the transmission reliability of video data is improved as compared with a multicast transmission scheme with no reception confirmation, but it takes a time to confirm the reception responses from the plurality of video receiving terminals, resulting in the reduction of transmission efficiency. In the reception response period in which the transmission and reception of the reception responses are performed, since the transmission and reception of the reception responses are performed in periods differing in the video receiving terminals, the length of the reception response period is a length corresponding to the number of the video receiving terminals. Particularly, in the case of performing video transmission in realtime, since trade-off occurs between video quality and the number of connectable terminals by an assignment ratio of a video data transmission time and a reception response period within the time of one frame, there is a problem that if video quality is to be maintained, the number of the connectable terminals is limited.
In order to avoid the aforementioned problem, Japanese Unexamined Patent Application No. 2008-17306 proposes a scheme of transmitting a packet for inquiring of an arbitrary receiving terminal about whether a transmission terminal can correctly receive multicast data.